Modern electronic prepress, offset and other types of printing operations write or record images for subsequent reproduction or read a prerecorded image at a predefined resolution rate. Such systems may write or record images or in the case of prepress systems, read prerecorded images on various media including, photo or thermal sensitive paper or polymer films, photo or thermal sensitive coatings, erasable imaging materials or ink receptive media mounted onto an image recording surface, or photo or thermal sensitive paper, polymer film or aluminum base printing plate materials, all used in image reproduction. Such media are mounted onto a recording surface which may be planar or curved.
In the case of prepress systems, the primary components include a recording surface, usually a drum cylinder and a scan mechanism disposed and movable within the drum cylinder. The system also includes a processor, with an associated storage device, for controlling the scanning mechanism. The processor and associated storage device may be housed within the system itself or separate from the system with appropriate interconnection to the system. The processor, in accordance with stored programming instructions, controls the scanning mechanism to write or read images on the medium mounted to the inner drum cylinder wall by scanning one or more optical beams over the inside circumference of the drum cylinder while the drum cylinder itself remains fixed.
The scanning and hence the recording are performed over only a portion of the cylinder inner circumference, typically between 120.degree. and 320.degree. of the circumference of the drum cylinder. The optical beam(s) are typically emitted so as to be parallel with a central axis of the cylinder and are deflected, by for example, a spinning mirror, Hologon or Penta-prism deflector so as to form a single scan line or multiple scan lines which simultaneously impinge upon the recording surface. The deflector is spun or rotated by a motor about an axis of rotation substantially coincident with the central axis of the drum cylinder. To increase the recording speed, the speed of rotation of the beam deflecting device can be increased.
Notwithstanding the type of system, whether prepress, offset printing or otherwise, being utilized, it is of primary importance that the images be recorded as close as possible to a desired location to ensure that appropriately positioned images are formed on the recording surface and hence the desired image is properly recorded. For example, in prepress systems, a synchronization error or beam printing error in a scan engine, a media positioning error, or other types of anomalies will cause errors in the positioning of the image on the medium. In offset printing type systems, misalignment of the plates forming a multiple plate image or of the paper feed or other anomalies will similarly cause image position errors which manifest themselves as a positioning error between respective images.
Often in prepress or printing operations, it is required that the same image be recorded numerous times in a precise location on the same or different sheets of media. In such cases, it is imperative that the image be repeatable within a tight position tolerance, e.g. less than a mil, on each sheet. If an anomaly exists in scan mechanism or emitter of a prepress or the rollers or feed of an offset printer, the images will not be properly positioned on each of the sheets of media and the result will be unacceptable. Errors of this type are commonly characterized as registration errors.
In image setting operations, it is customary for the positional repeatability to be verified with the media held stationary, to within a specified tolerance in two axes by repetitively exposing a test page containing fiducial marks, e.g. cross hairs, with a line image in multiple exposure fashion to form a register or registration mark which simulates multiple separate full sheet exposures. At each cross hair location, the x-y position error over the multiple exposures is estimated using a magnifying lens, e.g. a microscope, to detect the deviation between the centers of the overlaid images.
Because the minimum line width, i.e., a single pixel, of the image setter is typically much larger than the repeatability errors which must be measured, resolution of the position error measurement even with a microscope is compromised using the conventional approach. Also, by exposing multiple single pixel lines on top of each other, blooming of the exposed lines will occur and significantly increase the thickness of the line so as to further compromise the measurement resolution. Blooming may be reduced by lowering the individual exposure levels of the single pixel lines; however, this tends to result in a loss of images for a first number of exposures because there is insufficient energy for the respective exposures to create a visible mark on the media when the exposure level is lowered enough to eliminate the blooming effects. It will be understood that the loss of the initial images is yet another form of measurement resolution loss.
Additionally, single pixel lines are susceptible to transient position errors caused, for example, by random wobble. Such transient position errors may be interpreted to mean that positional repeatability is unacceptable when, in fact, statistically the errors may not represent the overall repeatability within a given area, such as the area of a halftone dot. On the other hand, if the line width is increased to several pixels to increase visibility, and provide a better statistical representation of the overall repeatability, it becomes much more difficult to detect misalignments, which often exceed the position error tolerance by an amount much less than the width of the line. Further still, using the conventional technique, variables such as media response, spot size, exposure setting, media processing, etc., may significantly affect the ability to detect repeatability errors because these variables will have a greater impact on the results obtained using conventional techniques than the actual position error to be detected.
More sophisticated techniques for detecting repeatability errors have been proposed which overcome at least some of the difficulties in the conventional approach. For example, one proposal is to use a highly sensitive moire pattern formed by superpositioning two separate patterns having slightly different spatial frequencies to serve as the register mark. When the patterns are properly aligned, a bright spot appears in the center of the register mark. However, when the patterns are misaligned, the bright spot is visually displaced. Although improving a viewer's ability to visually perceive a misalignment between the patterns, small misalignment errors remain difficult if not impossible to detect with the unaided eye or even a microscope. Further, the technique does not provide a way to quantify the extent or degree, i.e., the magnitude of the misalignment error. Additionally, from a prepress standpoint, the technique inherently requires a relatively large number of cycles to provide the necessary effect. The technique is not intuitive but rather requires a trained eye to determine with any level of certainty that an unacceptable misalignment exists based upon the position of the bright spot within the register mark.
Another technique which has been proposed for use in ion beam lithography utilizes alignment marks and apertures. The light radiating from the alignment marks is sensed and the intensity of the detected radiating light is measured to determine if the apertures and alignment marks are misaligned. This technique, although providing a relatively accurate means of detecting a misalignment and of obtaining a positional null, is impractical when it comes to image generation/replication operations requiring visual verification of acceptable alignment or quantification of the extent of the misalignment without the use of complex and expensive sensing devices.